The invention relates generally to fluid delivery systems, and more particularly, to an intravenous (IV) infusion system having a positive-pressure based mechanism for inducing fluid flow and a monitoring system for measuring the downstream resistance of the IV infusion system based on changes in pressure and flow rate.
IV infusion systems for infusing fluid to a patient typically include a supply of fluid for administration, an infusion needle or cannula, an administration set connecting the fluid supply to the cannula, and a flow control device. The administration set typically includes a flexible IV tube and a drip chamber. The cannula is mounted at the distal end of the flexible IV tubing for insertion into a patient's blood vessel or other body location to deliver the fluid to the patient.
The flow control device may be either gravity-pressure based or positive-pressure based. Gravity-pressure based flow control devices rely on the force of gravity for fluid flow. These devices may include an “IV controller” which interfaces with the IV tube. An IV controller is a device that automatically controls the flow rate of fluid through the IV tube by use of a pinching device that pinches the tube more or less to control the flow of fluid therethrough. The IV controller is usually responsive to a control signal which is typically generated by a flow sensor attached to the drip chamber. The flow sensor senses fluid drops falling in the drip chamber. The number of drops per unit time is counted and a flow rate calculated. If the calculated flow rate is greater than a desired flow rate, the controller adjusts the pinching device to lower the flow rate by pinching the tube further. Advantages of gravity administration sets include their relative simplicity and low cost. Relatively inexpensive tubing may be used such as polyvinyl chloride (“PVC”) tubing or similar type tubing. The pinching device comprises a relatively simple mechanical device under electrical control. IV controllers, however, are limited to gravity pressure, dependent upon the “head height” or “head pressure” of the administration fluid, which can be under 1 psi.
In certain situations the amount of pressure provided by a gravity-pressure based flow control device may be insufficient. In other situations, greater accuracy and precision of flow rates are required. In these situations a positive-pressure based flow control device is necessary. Positive-pressure based flow control devices exert a mechanical force on the fluid to establish fluid flow. One commonly used positive-pressure based flow control device is a linear peristaltic pump. A linear peristaltic pump is a complex device comprising several cams and cam-actuated fingers that sequentially occlude portions of the flexible tubing along a specially designed pumping segment to create a moving zone of occlusion. The peristaltic action forces the fluid through the tubing of the administration set to the cannula and into the patient. Because of its complexity and number of components, a linear peristaltic type pump is relatively expensive and may be undesirable in situations where cost containment is a factor. The pumping segment is also typically part of a disposable administration set and thus is relatively expensive.
Another type of positive-pressure based flow control device is a piston-and-valve-type device that uses a specially designed plastic cassette or cylinder device that interfaces with the piston and valve to control fluid flow. The cassette or cylinder is small in size and has precise dimensional requirements so as to provide accurate fluid flow control. Due to such requirements these devices are expensive to manufacture. The cassette or cylinder is also typically part of a disposable administration set and thus have an increased cost.
Another type of positive-pressure based flow control device includes a collapsible fluid treatment bag and an inflatable bladder. A fluid pump or other pressure source provides fluid, typically air, to the bladder. As the bladder inflates, pressure is applied to the collapsible fluid treatment bag. This pressure forces fluid through the tubing of the administration set to the cannula and into the patient.
During infusion events may occur that interfere with the proper administration of fluid to the patient, such as an occlusion of the administration line. It is desirable to detect these conditions as soon as possible so that they can be remedied. A commonly used technique for detecting such conditions and for evaluating the operating status of the IV infusion system is to monitor the pressure in the downstream portion of the fluid delivery tube. The “downstream” portion of the tube is typically thought of as the portion between the flow control device, such as the pinching device in a controller or the peristaltic fingers in a linear peristaltic pump, and the patient's blood vessel. An increase in the downstream pressure may be caused by an occlusion.
One measurement of downstream infusion system parameters that has proved useful is a measurement of resistance. Downstream resistance may be affected by a downstream occlusion, an infiltration of the cannula into the patient's tissue surrounding the blood vessel, a cannula that has become removed from the blood vessel, or others. By monitoring downstream resistance, an operator may be able to determine if any of the above events has occurred. Appropriate steps may be taken to remedy the situation sooner than with other monitoring approaches. It should be noted that when the cannula is in place in a patient's blood vessel, that blood vessel also contributes an effect to the flow and pressure in the tubing and is therefore considered part of the downstream resistance.
Sophisticated flow control devices monitor the downstream resistance of the infusion system by altering the flow rate through the tube and measuring the corresponding change in downstream pressure. The change in pressure over the change in the flow rate has been found to accurately indicate the resistive part of the downstream fluid impedance. In these systems, a pressure sensor is coupled to the infusion tube. The pressure sensor monitors the pressure existing in the downstream portion of the tube and produces pressure signals representing the detected pressure.
A disadvantage of these existing systems for detecting downstream resistance is that the pressure sensor must be coupled to the IV tube. Because of this, the pressure sensors must be capable of accurately detecting fluid pressure through an IV tube. Such sensors tend to be complex and expensive.
Hence, those skilled in the art have recognized a need for a simpler and less expensive positive-pressure based IV infusion system. Those skilled in the art have also recognized the need for an administration set using standard tubing and a standard flow monitoring system, such as a drip chamber, with a standard collapsible administration fluid container which may be accurately used with a positive pressure based IV infusion system. A need has also been recognized for a closed-loop positive-pressure based IV infusion system wherein flow rate and pressure can be monitored using standard administration set tubing and drip chamber devices for lowered cost. A further need has been recognized for a single integrated package design containing fluid source, flow sensing, flow control, pressure source, pressure sensing, and pressure control. The present invention fulfills these needs and others.